When receiving radio waves with antenna in mobile vehicles, the problem with multi-path distortion arises. This term is used as a designation for the disturbance which arises as a result of interference of radio waves. The interference is caused by that the transmitted waves are travelling somewhat different routes from a transmitter to a receiver. At the receiver, waves are received that may have been reflected for example by near by mountain walls and/or buildings, causing the transmitted waves to travel different distances. The interference results in that maxima and minima arise with distances within a magnitude which can be estimated within the area that the normal car represents. This problem has been solved by the use of a system with multiple antennas for receiving radio waves in cars, so called diversity.
A known diversity system for active compensation comprises multiple antennas, usually a main antenna and a sub-antenna, that are connected to an antenna selector unit which is a part of a radio unit. The antenna selector unit is in its turn connected to a tuner which is connected to a multi-path detector, which mediates an in-signal to said selector unit, which consists of a decision-making circuit which determines which of said antennas will serve as receiving antenna. If multi-path distortion is detected by the detector, the selector unit will immediately switch from one receiving antenna to another, in order to eliminate the disturbing phenomenon, if possible.
Problems may arise when installing a radio having the diversity function, if a sub-antenna is missing. The system may be adaptable via a manual change over switch, so that the installer may adapt said system on one hand to vehicles with a single antenna, on the other hand to vehicles with multiple antennas. If the installation is not correct, the selector will either, in a vehicle with a single antenna, to pendulate between the antenna and a nonexistent sub-antenna, or in a vehicle with multiple antennas, to avoid activation of the sub-antenna. In both cases, the reception will be less efficient than intended.
A similar problem occurs if damage arises on the sub-antenna or its antenna cable in a system having multiple antennas. This will result in that the antenna selector pendulates between the main antenna and the damaged sub-antenna, which leads to more inferior receiving conditions than if the selector was set to single antenna.
It would be technically possible to provide a selector unit which could electro-mechanically detect if an antenna connector is connected to the antenna port. A drawback with this automatic detection, is that a connector may be inserted by mistake, which connector has no connection with an antenna. Also, there will be no adaption of the selector unit, if some fault occurs on the sub-antenna. Also in these cases, the selector unit will try to select the nonexistent respectively the damaged sub-antenna, when multi-path distortion has been detected.
U.S. Pat. No. 5,216,434 describes a receiver unit with an antenna selector for a radio receiver with diversity function. Each antenna is connected to a source of current. A fault detecting circuit is connected to the receiver unit, which circuit controls the potential at the current supply connectors for the antenna. At interruption of the antenna feeder cable, the detected potential is changed which is registered as a fault and is indicated by means of a LED. Then it is possible to manually switch off the multi-path function at the receiver unit. Thus, this prior art solution requires that the user both must know the reason for the fault indication, and also must be able to execute the manual switch off.